1. Field of the Invention
The present invention relates to an IGBT which is operated by a predetermined voltage applied to a gate electrode and to a method of fabricating the same.
2. Description of the Background Art
FIG. 24 is a cross-sectional view of a conventional insulated gate bipolar transistor (hereinafter referred to as an IGBT). An n.sup.+ epitaxial layer (hereinafter referred to as an n.sup.+ layer) 2A is formed on the upper major surface of a p.sup.+ substrate 1. An n.sup.- epitaxial layer (hereinafter referred to as an n.sup.- layer) 2B is formed on the n.sup.+ layer 2A. In the surface of the n.sup.- layer 2B is formed a p well region 3 provided by the diffusion of p-type impurities. In the surface of the p well region 3 is formed an n.sup.+ diffusion region 4 provided by the selective diffusion of n-type impurities.
A gate oxide film 5 is formed over the surface of the p well region 3 which is between the n.sup.+ diffusion region 4 and n.sup.- layer 2B. A gate electrode 6 made of polysilicon is formed in the gate oxide film 5. An emitter electrode 8 made of metal is provided in contact with the n.sup.+ diffusion region 4 and p well region 3. The emitter electrode 8 and the gate electrode 6 are insulated from each other through an interlayer oxide film 7. On the lower major surface of the p.sup.+ substrate 1 is formed a collector electrode 9 made of metal.
The IGBT thus constructed operates as follows. When the emitter electrode 8 and the gate electrode 6 are at the same potential and positive bias is applied to the drain electrode 9, a depletion layer extends from a pn junction formed by the p well region 3 and n.sup.- layer 2B toward the n.sup.- layer 2B. This holds the voltage in the OFF-state. Increasing the positive bias, the depletion layer extends up to the n.sup.+ layer 2A and then stops, the depletion layer is prevented from reaching the p.sup.+ substrate 1 and being brought into a reach-through state.
When the gate electrode 6 is positive-biased relative to the emitter electrode 8, n-inversion occurs to form an n-channel in the p well layer 3 immediately under the gate electrode 6, so that electrons flow from the n.sup.+ diffusion region 4 through the n-channel to the n.sup.- layer 2B. The voltage retention in p the well region 3 and n.sup.- layer 2B goes eliminated, and the potential between the p.sup.+ substrate 1 and the n.sup.+ layer 2A is in the forward direction so that holes flow from the n.sup.- layer 2B to the p well region 3. The modulation by the holes injected from the p.sup.+ substrate 1 increases the concentration of electrons in the n.sup.- layer 2B, so that a low ON-resistance is provided. When the gate electrode 6 and the emitter electrode 8 are made to be again at the same potential, the n-channel disappears and the voltage retention between the p well region 3 and n.sup.- layer 2B restarts. The potential between the p.sup.+ substrate 1 and n.sup.+ layer 2A is no longer in the forward direction, and the injection of holes stops. The holes which have already been injected in the n.sup.- layer 2B successively reach the p well region 3, and the resultant tail current causes power loss. To reduce the tail current, a recombination center is introduced by doping the n.sup.- layer 2B with heavy metal or by irradiation of electron beams and the lifetime of carriers is shortened. For the low ON-resistance, in general, it is effective to increase the resistivity of the n.sup.+ layer 2A, to decrease the thickness thereof, or to prolong the lifetime of the carriers. A device of low ON-resistance, however, is easily broken down by an excessive collector-emitter voltage, if applied, in the ON-state because of an excessively large current density.
In the conventional IGBT which is optimized under normal use conditions and has the low ON-resistance, there arises a problem that an excessive collector-emitter voltage (e.g., short-circuit), if applied in the ON-state, increases forward bias between the p.sup.+ substrate 1 and the n.sup.+ layer 2A and the amount of injected holes, and causes the excessively increased current density, resulting in the breakdown of the device.
On the other hand, another problem is that, when the excessive increase in current density is restrained to prevent the breakdown, the current density under the normal use conditions decreases, resulting in an increased ON-resistance.